Levothyroxine is a thyroid hormone product which is commercially available in 11 different dose strengths, of which only 5 strengths are permissible on the hospital formulary. The purpose of this prospective, randomized controlled trial is to determine whether limiting dosage strengths of levothyroxine, affect physicians' ability to effectively manage patients. This trial will assign participating endocrinologists from the NICHD and NIDDK outpatient clinics to restrictive (25, 50, 100, 125, and 150 ug dosage formulations permitted) or non-restrictive (25, 50, 75, 88, 100, 112, 125, 150, 175, 200, and 300 ug dosage formulations permitted) levothyroxine prescribing groups. Success in achieving therapeutic objectives as measured by thyroid function tests, prescribing patters, patient compliance (by survey), frequency of thyroid function studies and clinic visits, medication distribution accuracy, and inventory cost will be compared statistically between groups. Preliminary results from 241 patients of 33 endocrinologists suggest that the differences in therapeutic success between restricted and non-restricted thyroid patients were not clinically significant. The compliance, frequency of thyroid function tests, clinical visits and medication errors were also similar. The restricted formulary, however, was significantly more often associated with complex levothyroxine dosing regimens. Further, the inventory and prescription costs were slightly lower with use of the non-restrictive formulary system.